1. Field of the Invention
The present invention relates to a method for improving system throughput of service generating small amount of traffic such as VoIP (Voice over IP) in 3GPP LTE (3rd Generation Partnership Project Long Term Evolution) system.
2. Description of the Related Art
The mobile communication system has evolved into a high-speed, high-quality wireless packet data communication system to provide data services and multimedia services beyond the early voice-oriented services. Recently, various mobile communication standards, such as High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA), both defined in 3rd Generation Partnership Project (3GPP), High Rate Packet Data (HRPD) defined in 3rd Generation Partnership Project-2 (3GPP2), and 802.16 defined in IEEE, have been developed to support the high-speed, high-quality wireless packet data communication services.
Such recent mobile communication systems adopt Adaptive Modulation and Coding (AMC) and channel sensitive techniques to improve transmission efficiency. With AMC, the transmitter can control the data amount according to channel state. That is, when the channel state is bad, the data rate is decreased to math a predetermined error rate, and when the channel state is good, the data transmission rate is increased to match another predetermined error rate. In this way, the mobile communication system can transmit large amount of information efficiently.
With the channel sensitive scheduling resource management method, the transmitter can serve the user having superior channel state first selectively among multiple users and thus increase system throughput as compared to the general channel allocation and serving method. For example, the AMC and channel sensitive scheduling are the techniques for using the most appropriate modulation and coding scheme at the most efficient timing based on the partial channel state information fed back by the receiver.
There has been many researches done to adopt Orthogonal Frequency Division Multiple Access (OFDMA) to next generation communication systems in place of Code Division Multiple Access (CDMA) that has been used in 2nd and 3rd Generation mobile communication systems. The standardization organizations such as 3GPP, 3GPP2, and IEEE are developing standards for enhanced system based on the OFDMA or modified OFDMA. It is known that OFDMA promises to increase system capacity as compared to CDMA. One of the factors affecting the increase of system capacity in an OFDMA system is the use of frequency domain scheduling. As the channel sensitive scheduling technique uses the time-varying channel for capacity gain, it is possible to increase the capacity gain with frequency-varying channel characteristic.
LTE adopts Orthogonal Frequency Division Multiplexing (OFDM) in Downlink (DL) and Single Carrier Frequency Division Multiple Access (SC-FDMA) that are both capable of scheduling in frequency domain.
The AMC and channel sensitive scheduling are the techniques that are capable of improving the transmission efficiency in the state where the transmitter has acquired information enough on the transmit channel. In FDD (Frequency Division Duplex) mode where the transmitter cannot estimate the state of the transmit channel based on the receive channel, it is designed for the receiver to report the information on the transmit channel to the transmitter. In case of TDD (Time Division Duplex) mode where the transmit channel state can be inferred from the receive channel state, the transmit channel state report from the receiver to the transmitter can be omitted.
Although the mobile communication system evolves to support high speed high quality radio packet data communication, voice communication is still one of the main services. VoIP (Voice over Internet Protocol) is the service for supporting efficient voice communication in the communication system designed for radio packet data communication. The voice communication service has a characteristic in that a certain amount of traffic takes place at a certain interval and very sensitive to delay. In case of the traffic not sensitive to time delay, the scheduler can schedule the data on appropriate radio resource with temporal margin so as to expect high gain with the AMC and channel sensitive scheduling. In case of the delay sensitive traffic, however, the scheduler has to schedule the data without enough temporal margin and, as a consequence, it is difficult to expect the large gain of channel sensitive scheduling. Also, if a certain amount of traffic takes place regularly, the AMC which changes the modulation and coding scheme depending on the data amount becomes useless. In such cases, the control signals for indicating the modulating and coding scheme that are transmitted by using the AMC and channel sensitive scheduling techniques causes consummation of relatively large amount of resource as compared to the data amount to be transmitted.
By taking notice of these features of VoIP, a new resource allocation scheme appropriate for VoIP, referred to as SPS (Semi-Persistent Scheduling), is defined in LTE. SPS is designed to prevent the control signal from being transmitted for every data transmission such that the resource is allocated automatically at the interval of the traffic occurrence with the initially configured modulation and coding scheme until a resource release signal is transmitted.
FIG. 1 is diagram illustrating a resource unit defined for downlink in a LTE system.
Referring to FIG. 1 LTE adopts OFDMA in downlink. The resource is allocated in unit of resource block (RB) 110. An RB 110 is generated from 12 subcarriers on frequency axis and a subframe on time axis. A subframe has 1 msec duration and consists of 14 OFDM symbols. 10 subframes constitute a radio frame.
The basic unit of radio resource is resource element (RE). An RE 112 is defined by one subcarrier on frequency axis, one OFDM symbol on time axis, and one virtual antenna port on spatial axis. This means that one RE carrier a modulation signal. In case of allocating resource in unit of RE, too much information amount is required for indicating allocated resources and thus resource allocation is performed in unit of Resource Block (RB).
FIG. 2 is a diagram illustrating an RB composed of a plurality of REs arranged with different purposes in LTE downlink.
Referring to FIG. 2, an enhanced Node B (eNB) transmits predetermined reference signal (RS) at predetermined positions in an RB for channel estimation. A User Equipment (UE), which knows the position of the resource designated for the signal, estimates channel according to the received RB. Reference number 120 denote the RE used for common RS. FIG. 2 shows the RB configured with common RS in case that 4 antenna ports are used.
One RB consists of 168 (=12×14) REs and, when the number of transmit antennas is 4, 24 REs are assigned for common RSs. In order to indicate resource allocation, Physical Downlink Control Channel (PDCCH) is transmitted in n OFDM symbols duration at the beginning of a subframe which is referred to as control region 122. Here, n can be 1, 2, or 3. Since the control region consists of 3 OFDM symbols in the example of FIG. 2, n=3. The size of the control region can vary every subframe and indicated by Physical Control Format Indicator Channel (PCFICH) at the first OFDM symbol of each subframe.
In the control region 122, physical ARQ Indicator Channel (PHICH) carrying ACK/NACK signal for Physical Uplink Shared Channel (PUSCH) is defined to support HARQ process. That is, the control channel region carries RS, PDCCH, PCFICH, and PHICH. Reference number 124 denotes RE designated for control signal transmission. Reference number 126 denote RE designated for PUSCH transmission carrying scheduled user data.
PDSCH cannot be transmitted in the control region 122. Accordingly, in the exemplary case of FIG. 2 where the number of transmit antenna ports is 4 and the control region consists of three OFDM symbols, the number of REs that can be used for PDSCH transmission in an RB is 116.
Table 1 summarizes the number of REs available for PDSCH transmission in one RB according to the number of eNB's transmit antennas, whether the dedicated RS is defined, and length of the control region.
TABLE 1# of transmitSize of controlNumber ofConditionantenna portsDedicated RSregionPDSCH REs11Not defined115021Not defined213831Not defined312641Defined113851Defined212661Defined311472Not defined114482Not defined213292Not defined3120102Defined1132112Defined2120122Defined3108134Not defined1136144Not defined2128154Not defined3116164Defined1124174Defined2116184Defined3104
The number of REs determined which is by referencing table 1 is available in case of using normal Cyclic Prefix (CP) and system bandwidth is greater than 10 RBs. This is because the number of OFDM symbols constituting a subframe using an extended CP decreases to 6 and the size of the control region is configured with 2, 3, or 4 rather than 1, 2, or 3 when the system bandwidth is equal to or less than 10 RBs.
Reference number 130 denotes a principle of frequency first mapping for PDSCH. The modulation symbols streams of PDSCH are arranged in ascending order direction of subcarrier indices on the frequency axis. Once all of the subcarriers in an OFDM symbol are assigned, and then subcarriers in next OFDM symbol are assigned for PDSCH resource.
In an RB, dedicated RS can be defined additionally. The dedicated RS means the RS, when beamforming is applied for a scheduled user, to which the same beamforming is applied as to the PDCCH. The dedicated RS is mapped to 12 REs per RB.
FIG. 3 is a block diagram illustrating a configuration of an eNB transmitter in a legacy LTE system.
FIG. 3 shows the configuration of normal eNB transmitter using spatial multiplexing (SM) in case of transmitting two codewords (CWs).
Transport Block (TB) denotes the information signal delivered from a higher layers to a physical layer to be transmitted through PDSCH. In case of supporting SM, up to two TBs can be transmitted on the same resource. Reference number 200a denotes TB1 for the first CW, and reference number 200b denotes TB2 for the second CW.
TB1 200a and TB2 200b channel coded by the channel coders 202a and 202b, scrambled by the scramblers 204a, 204b, and modulated by the modulators 206a and 206b so as to be converted to modulation signal streams. In case that no SM is used, TB1 200a does not exist such that the processes 202b, 204b, and 206b for TB1 200a are omitted.
The one or two modulation signal streams are converted to modulation signal streams per spatial layer to which precoding is applied by the precoder 210 through layer mapper 208. In case of SM the number of CWs is limited to 2 but four spatial layers are allowed an thus it is necessary to define the arrangement for this.
In case of using transmit diversity, the number of spatial layers is 2 or 4 while the number of CWs is 1, and precoding means the transmit diversity coding. The precoding can be categorized into one of transmit diversity coding, open-loop precoding, and closed-loop precoding.
The precoder 210 can converts the modulation signal streams per spatial layer to signal streams to be transmitted through respective transmission antenna ports 2116c and 216d. The precoded signal streams are mapped to REs corresponding to antenna ports by RE mappers 212c and 212d, converted to OFDM symbols by OFDM signal generator 214c and 214d, and then transmitted through respective antenna ports 216c and 216d. 
FIG. 4 is a diagram illustrating a resource unit defined in uplink of a legacy LTE system.
Referring to FIG. 4, LTE adopts SC-FDMA in uplink, and the basic unit of radio resource is RE as in downlink. An RB 400 is generated from 12 virtual subcarriers and one subframe on time axis. The subframe has 1 msec duration and consists of 14 OFDM symbols as in downlink. The RE 402 is defined by a virtual subcarrier in an SC-FDMA symbol.
SC-FDMA applies Discrete Fourier Transform as precoding at step prior to OFDM signal generation. Accordingly, the RE mapped designated for a modulation symbol does not means a subcarrier but is referred as virtual subcarrier. In case of allocating resource in unit of RE, too much information amount is required for indicating allocated resources and thus resource allocation is performed in unit of Resource Block (RB).
FIG. 5 is a diagram illustrating resource arrangement per purpose in an RB in uplink of legacy LTE.
Referring to FIG. 5, RS can be defined in unit of RE in SC-FDMA. Accordingly, a specific SC-FDMA symbol is entirely used for RS. The uplink RS is defined in the form of dedicated RS for per-user uplink channel estimation and used for demodulation so as to be referred to as Demodulation RS (DM RS).
Uplink Multi-User Multiple Input Multiple Output (MU-MIMO) means uplink Space Domain Multiple Access (SDMA). If the dedicated RSs that are orthogonal between user are received, the eNB receiver applies spatial filter appropriate for the per-user spatial channel response to discriminate among PUSCHs transmitted to different users.
The fourth and eleventh SC-FDMA symbols of a subframe 510 are SC-FDMA symbols 514 for transmitting the dedicated RS. By transmitting the per-user orthogonal signals as dedicated RS, it is possible to support uplink MU-MIMO. Other SC-FDMA symbols 516 are the resource available for PUSCH transmission. Among the total 168 (=14×12) RBs are defined in LTE uplink, the number of REs available for PUSCH transmission is 144 (12×12) with exception of resource assigned for RS.
In case of the subframe including Sounding RS (SRS) transmitted for uplink channel state at eNB, the last SC-FDMA symbol of a subframe is used for SRS transmission. Accordingly, the number of PUSCH REs is 132 (=11×12) per RB. Here, the number of REs is for a normal CP subframe. In case of using the extended CP, the number of SC-FDMA symbols constituting the subframe decreases and, as a consequence, the number of RE varies.
FIG. 6 is a block diagram illustrating a configuration of a UE transmitter in a legacy LTE system.
Referring to FIG. 6, the TB 620 is channel coded by a channel coder 622, scrambled by a scrambler 624, and modulated by a modulator 626, so as to be output as a modulation symbol stream. The modulation symbol stream is transform-precoded by a transform precoder 628 as DFT precoder, mapped to PUSCH RB by an RE mapper 630, and then converted to SC-FDMA signal by a SC-FDMA signal generator 632 to be transmitted through a UE transmit antenna 634.
FIG. 7 is a block diagram illustrating a configuration of an eNB receiver in a legacy LTE system.
Referring to FIG. 7, the signal received by the receive antenna 710 processed by a SC-FDMA signal receiver 712 and then RE-demapped by an RE demapper 714. In case that the eNB has multiple receive antennas, it is assumed that an antenna combiner is included in the SC-FDMA signal receiver 712. The signal separated to decode the DFT precoding applied at the UE for generating the SC-FDMA signal is processed by a transform-precoding decoder 716 as an Inverse DFT (IDFT), demodulated by a demodulator 718, descrambled by a descrambler 720, and channel-decoded by a channel decoder 722 so as to be recovered as the TB 724 transmitted by each user.
FIG. 8 is a diagram illustrating resource allocation according to channel state of UE in conventional system.
Referring to FIG. 8, two UEs 802 and 804 are connected to a eNB 800. The UE 804 is located close to the eNB 800 as compared to the UE 802 such that the average channel state of the UE 804 is superior to that of the UE 802.
If the channel response of UE 802 is good enough to apply a high order modulation and coding scheme, it is possible to allocate the resource small in amount to the UE 802 for transmitting the same data amount as the UE 804. For example, if the 16QAM (16 Quadrature Amplitude Modulation) is applied to the UE 802 while QPSK (Quadrature Phase Shift Keying) to the UE 904, the PDSCH or PUSCH for UE 802 carries 4 coded bits per RE while the PDSCH or PUSCH for UE 802 carries 2 coded bits per RE. If the same channel coding is applied and the same data amount is transmitted, the UE 804 needs only half of the resource allocated to the UE 804.